1. Field of the Invention
The present invention relates in general to disk storage system and more particularly, to an apparatus and method for generating servo information so as to provide gain linearization for positioning the read head of a hard disk drive.
2. Description of the Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is recorded on concentric tracks on either surface of one or more magnetic recording disks. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms, which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads. The read/write heads must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
To accurately write and read data, it is desirable to maintain the head on the center of the track. To assist in controlling the position of the head, each sector of the disk typically contains a number of servo bits accurately distributed evenly on each track. The raw signals produced by the servo bits are typically demodulated into a position signal which is utilized to determine the position of the head relative to the track, and to move the actuator arm if the head is not located on the track centerline.
Dual element transducers have been utilized in hard disk drives because they have greater aerial densities than signal element transducers. Dual element transducers include a single write element and a separate read element, which is constructed from a magneto-resistive material. Such dual element transducers are commonly referred to as magneto-resistive (MR) heads. Because the centerline of read head is different from the centerline of write head, an adjustment of read/write head position in track width is needed to cope with reading and writing separately. This sub-track position information becomes important to obtain reliable reading and writing. However, in most disk drives, the actual variation of the servo burst signals with respect to the track position for a MR head is non-monotonic and non-linear in nature and varies greatly between different heads.
FIG. 1A is a graph illustrating the variation of the servo burst signals A, B, C, D with respect to the position of the read head of a disk drive under near-ideal conditions. FIG. 1B is a graph illustrating the variation of the difference between servo burst signals (Axe2x88x92B) and (Cxe2x88x92D) with respect to the position of the read head of a disk drive under near-ideal conditions. FIG. 1C is a graph illustrating the variation of the difference between (Axe2x88x92B)xe2x88x92(Cxe2x88x92D) and (Axe2x88x92B)+(Cxe2x88x92D) with respect to the position of the read head of a disk drive based on (Axe2x88x92B) and (Cxe2x88x92D). FIGS. 1A to 1C illustrate the servo burst signal based on three typical techniques used to provide correlational information between the amplitude of the position error signal and the distance between the head and center of the track.
In above cases, each of the servo burst signals used varies monotonically with the track position of the read head. Although such monotonic variation permits the servo system to correct the off-track position of the read head, the useful linear range of the servo signal is limited in each case. For example, in FIG. 1A, the portion of the servo signal beyond +/xe2x88x9210% off the track centerline, is non-linear. One solution to this problem is to combine the user of the linear portions of two servo signals while the read head is over one particular track. For example, the linear portions or segments of the (Cxe2x88x92D), the (Axe2x88x92B) and the (Axe2x88x92B)xe2x88x92(Cxe2x88x92D) signals (as shown in FIG. 1B) or the linear portions of the (Axe2x88x92B)+(Cxe2x88x92D) and the (Axe2x88x92B)xe2x88x92(Cxe2x88x92D) signals (as shown in FIG. 1C) are relied upon to provide a sufficiently wide linear range for generating servo information over a single track. However, as shown in FIGS. 1B and 1C, the linear segments are not continuous. As a result, there are certain regions in which no servo information is available. In addition, for systems that rely on the use of a combination of the (Axe2x88x92B), (Cxe2x88x92D), (Axe2x88x92B)+(Cxe2x88x92D) and (Axe2x88x92B)xe2x88x92(Cxe2x88x92D) signals (for example, as shown in FIG. 2D), the slope xcex2 for the (Axe2x88x92B) signal in FIG. 2D is typically different from the slope xcex1 for the (Axe2x88x92B)+(Cxe2x88x92D). As a result, for systems that rely on such use of combination servo information, the resulting positioning information from one linear segment is inconsistent with that from another linear segment.
In addition, the ratio of the read head width with respect to track width is a significant factor in providing a linear range of burst signals. FIG. 2A is a graph illustrating the variation of A, B, C and D Burst provided using a narrow read head while FIG. 2B is a graph illustrating the variation of (Axe2x88x92B), (Cxe2x88x92D), (Axe2x88x92B)+(Cxe2x88x92D) and (Axe2x88x92B)xe2x88x92(Cxe2x88x92D) provided using narrow read head. Such narrow read heads typically have a read head width to track width ratio, x, of less than 0.5. As shown in FIGS. 2A and 2B, the use of narrow read heads, while providing relatively linear servo signals, also result in the existence of a dead zone, in which servo signal outputs are unavailable. In addition, it takes a longer time for a narrow read head to read the servo bursts and also for the read head to settle if the servo bursts are read using a narrower head. FIG. 2C is a graph illustrating the variation of A, B, C and D Burst with a near-normal actual read head, while FIG. 2D is a graph illustrating the variation of (Axe2x88x92B), (Cxe2x88x92D), (Axe2x88x92B)+(Cxe2x88x92D) and (Axe2x88x92B)xe2x88x92(Cxe2x88x92D) with respect to a near-normal (non-narrow) and actual read head. Such non-narrow read heads typically have a read head width to track width ratio x of greater than 0.5. As shown in FIGS. 2C and 2D, the use of such near-normal heads typically results in providing servo signal bursts having different slopes xcex1 and xcex2. In particular, the slope xcex1 is determined by the distance from X to Y, which varies with ratio of the head width to track width. The slope xcex2 is similarly determined. Thus, if the tolerances for the head dimensions are not strictly met, it will result in variations of not in the servo signal linearity but also in the slope consistency from signal to signal.
Accordingly, there is a need in the technology for overcoming the above described problems so as to reduce the manufacturing cost of read heads through relaxation of head dimension tolerances and also to provide servo information which varies linearly with respect to the track position of the MR read head.
A method and apparatus for generating a position information signal for the head of a hard disk drive. The disk has a track which contains a plurality of servo bursts that allow the head to be centered with the centerline of the track. The servo bursts are first sensed and an upper and a lower threshold values, based on the values of servo bursts, are generated. A position error signal based on the upper and lower threshold and the values of the servo bursts are generated and stored in a memory device.